= Mutual inductance of a straight wire and square coil
{title2=$\mathcal M=\frac{\mu_0Na}{2\pi}\ln\frac{s+a}{s}$}
An N-turn square coil of side a lies in the plane of a long straight wire, with nearest edge at perpendicular distance s and parallel to the wire. Integrating $B=\mu_0I/(2\pi r)$ across the coil gives flux-linkage magnitude $\Lambda=\mu_0NaI\ln[(s+a)/s]/(2\pi)$. Its coefficient of I is the <mutual inductance> magnitude; its sign depends on the selected loop normal and current directions. <Flux> through one turn and N-turn <flux> linkage must be distinguished.
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